The project goal is the evaluation and treatment of thyroid disorders. Patients with thyroid function abnormalities (hyperthyroidism, hypothyroidism) are enrolled. Patients undergo routine history and physical examination, standard endocrine blood and urine tests, a standard TRH test (using a preparation under IND), thyroid nuclear medicine scans, thyroidal radioiodine (RAI) or technetium (99mTc) uptake measurements, as well as X-ray, computed tomography (CT) or magnetic resonance imaging (MRI) studies or other standard diagnostic procedures, as clinically indicated. Patients are screened for entry into other research studies including the effects of thyroid hormone on the cytochrome p450 and p-glycoprotein activity. Another area of study is the methods for preoperative diagnosis and therapy of thyroid cancer. Study subjects include adults and children with thyroid nodules or cancer requiring diagnostic fine needle aspiration biopsy, surgery, radioiodine scanning or therapy for persistent or recurrent disease. In this natural history protocol, the use of methods for follow-up of patients using radiopharmaceutical tracers such as I-131, I-123, 201Tl chloride, 99mTc-Sestamibi, 111In-pentetreotide and 18-FDG PET will be evaluated. The limitations and significance of serum thyroglobulin (Tg) measurement for diagnosing tumor recurrence are assessed as is the risk/benefit ratio of already established methods of administering I-131 therapy including the impact of pre-treatment dosimetric calculations and administration of adjuvants such as recombinant human TSH and lithium. The use of radioactive iodine in the treatment of patients with stage I or II thyroid cancer who are at low risk for mortality is controversial. In a randomized, controlled clinical trial, the effects of lithium carbonate or placebo on the rate of successful ablation of thyroid remnant will be asssessed. By increasing the biological half-time of I-131 through its effects on thyroid hormone release, it is hypothesized that adjuvant lithium administration will improve the efficacy of low dose radiodine (30 mCi). The goal is to obtain satisfactory ablation of thyroid with minimum radiation exposure. The study will recruit 100 low risk patients over the age of 16 years with differentiated, non-medullary thyroid cancer. Samples of benign nodules and cancer tissue specimens for research studies will be collected to assess new immunohistochemical markers, and other techniques to characterize tumors for correlation with response to therapy and prognosis. Blood specimens are collected for future clinical and research studies in both the hypothyroid and euthyroid state. Coded clinical data are entered in the National Thyroid Cancer Registry in selected patients. This study also serves to screen patients for participation in other protocols.